The patterns of excitability of alpha motoneurons in patients with various psychiatric diagnoses are studied. In particular, the recovery curve of excitability to reflexively-evoked muscle responses (H-reflex) are compared among patients and non-patient volunteer controls. The effects of medication are studied by comparing pre-treatment excitability patterns with post-treatment curves. Significant alterations have been found in the patterns of excitability of some schizophrenic patients. The peak of "secondary facilitation" of the recovery curve tends to be reduced in young, poor prognosis schizophrenic subjects. The secondary facilitatory peak is higher than normal in chronic schizophrenic subjects - particularly those with tardive dyskinesia. The effects of medication on recovery curves are highly variable between individuals and may reflect basic inter-individual differences in the responsiveness of neuronal systems to medication or to differences in absorption and metabolism of medication. Sources of variation in response to medication such as genetic, demographic, and clinical variables are being studied. In addition, the possible relationship of reflex excitability alterations to previously described muscle pathology in psychotic patients is explored. Altered excitability of motoneurons is associated with an increased incidence of muscle pathology such as elevated serum creatine phosphokinase activity and increased numbers of atrophic muscle fibers.